I have a 3 year old water heater that has started to overflow through the relief valve. I belief this is the same issue the last water heater had but cannot remember. Anyway, here is what I know

*I have a backflow prevention valve that is tested annually and has always passed (last tested in June).
*I have an expansion tank. In my non-expert opinion, I hear 2 different sounds while hitting it from top to bottom.
*The water is definitely coming from the relief valve and is not leaking on its own. Seems to only be discharging water when hot water is run in washing machine / dishwater. I ran the washing machine on warm and water came out. I ran it on cold and no water came out.
*Heat is set to hot which is the medium of the dial which goes from low to super hot.
*This past year, I have noticed a my dishwater has been leaving water marks on glasses and the dried area around the water heater has suddenly become white this year (lime deposit I am assuming).
*I live in a townhouse and both my direct neighbors had their water heaters go at the same time about 1.5 months ago and both heaters were ~5 years old.

So basically, I need to know what to do. Both of my neighbors are on their 3rd water heater in 10 years which is concerning me. If I call my water provider what do I ask them? Do I need to call a plumber? Is there something I can do to fix the problem?

Do you know what your water pressure is or would you like help finding out how to test the psi? We need this to start with, otheroptions are the safety valve is doing what it supposed to, your expansion tank is not working, someone tested your safety and now it will not reseal...

If I call my water provider what do I ask them? Ask them what the street PSI us? Do you have a PRV, (pressure reducing valve) (see image) where the water enters your house?
Do you have a expansion tank connected to the heater?
Do you flush your heater on a regular basis?
Back to you, Tom

I still would like to know the psi in your home. The water provider may say they are putting out 45 psi but it can be much higher, or lower, in your home do to other issues.

I do have a PVR.
I do have an expansion tank connected and it makes 2 sounds when tapped which from what I have read indicates it is still good.
I do not regularly flush the heater.

I do not know what the psi coming in is. Wouldn't the PVR make this irrelevant? When plumbers come to check the backflow prevention value, wouldn't they check also to see what the pressure coming in is too?

Prv fail all the time as can expansion tanks. Check the psi

I do not know what the psi coming in is. Wouldn't the PVR make this irrelevant? It would if it were set at the correct PSI. But you don't know what that is do you? Why not have your plumber cat a tee in front of the PRV valve so you can install a gage to see for yourself? Until then I'll continue to believe the root cause of your problem is too high a PSI in your house. Good luck, Tom